The present invention relates to a method of preparing sodium monosulfide by means of reacting sodium and sulfur under a protective gas.
Sodium monosulfide, Na.sub.2 S, is an important reagent for introducing sulfur into organic molecules. Several methods are known according to which this compound can be prepared. It can be obtained for example, by reacting sodium salts with hydrogen sulfide in aqueous or alcoholic solutions or by the reduction of sodium sulfate with carbon or hydrogen. The known methods have the problem that products contaminated with reactants always accumulate which must be separated from the impurities by being dissolved in suitable solvents and by filtering.
Since the elements sodium and sulfur react extremely vigorously with one another (enthalpy of formation for Na.sub.2 S: .DELTA.HB=-389.1 kJ/mole), the direct preparation of Na.sub.2 S from the elements which are commercially available in great purity was not feasible for industrial purposes in the past.
Moisture-free Na.sub.2 S in sufficiently pure form was only obtainable by a process of dewatering the hydrate Na.sub.2 S . 9H.sub.2 O under an atmosphere of hydrogen, according to which this hydrate had to be prepared by means of reacting sodium hydrogen sulfide with NaOH in a polar solvent (Kirk-Othmer, 3d edition (1982), vol. 18, pp. 793-847, especially pp. 803 and 809).
It is also known from the art; namely, DE-PS 34 36 698 that sodium polysulfides can be prepared from the elements sodium and sulfur in which the sodium and the sulfur are charged under a protective gas in a stoichiometric proportion corresponding to the desired polysulfide in an alternatingly manner into a melt of a polysulfide placed in a reaction receiver. Vigorous agitation is carried out. The portions of reactants are measured in such a manner that during the charging of sodium the reaction mixture remains in the state of an agitatible suspension and that during the charging of the sulfur the latter is allowed to react completely in each instance to form a polysulfide of a higher sulfur content.
It would also be basically possible according to this method to recover sodium monosulfide by providing the required stoichiometric proportion for the bound sulfur and the sodium. However, the method referred to above requires a melt as the reaction medium which consists at least at the final stage of the reaction of Na.sub.2 S alone. Therefore, an economically feasible reactor material would hardly be available as a consequence of having to withstand the very high melting point of this compound (1180.degree.-1200.degree. C.) and on account of the strong chemical aggressivity of the melt.